Thanks for writing in! When using X theme and have successfully validated X theme, you should no longer be asked to validate Cornerstone because it will be validated automatically. Please be advised that Cornerstone will only ask you to validate it if:
1.) If you are not using X and activated Cornerstone as a stand alone plugin
2.) If you have installed X incorrectly. Cornerstone will look for the x folder. If you have renamed it, or incorrectly uploaded the folder then it will result to this issue.
There are times that you have uploaded the wrong zip file and it will result to this folder path which is not correct:
...\wp-content\themes\x\x\
Please make sure that x is in the correct path.
...\wp-content\themes\x\
And by the way, regarding your css, you might need to resave your settings to resolve the issue. For more details, please check this out: